The present invention relates generally to a method and apparatus for cam profile mode switching. More particularly, the apparatus of the present invention relates to a two-step roller finger follower for cam profile mode switching.
Vehicle manufacturers have different goals for various vehicle platforms or models. The primary goal for one particular model may be to provide relatively high fuel economy, and for another model the goal may be to provide relatively high engine power output. The goal of providing relatively high fuel economy can be accomplished through the use of a cam having a relatively low lift and short duration lift profile, whereas high engine power is provided by the use of a cam having a higher lift and longer duration profile. Thus, the goals of high fuel economy and high power often involve conflicting design choices, and an acceptable tradeoff must be reached between the two competing goals for a particular vehicle model.
Generally, high lift long duration output cams are designed to provide high power output at high engine operating speeds. However, such high lift long duration output cams typically result in-decreased engine idle quality and reduced low speed torque and drivability due to reduced air mass charged into the cylinder which is associated with the relatively long overlap duration of the opening and closing of the engine valves. In effect, cams which are designed to increase volumetric efficiency at high engine operating speeds typically reduce volumetric efficiency at lower engine operating speeds. A high lift long duration cam improves volumetric efficiency at high engine. operating speeds by increasing the flow rate past the valve. Additionally, the longer duration lift provides more time in which to fill the cylinder with air. Further, the relatively late closing of the intake valve associated with a long duration cam takes advantage of the inertial effects of the intake charge to further increase high speed volumetric efficiency.
Conversely, a low lift short duration cam is best suited for low engine operating speeds due to improved intake charge-velocity, which improves volumetric efficiency. The increased intake charge velocity also creates a more homogenous mixture that improves combustion by increasing either swirl or tumble. Additionally, the shorter duration cam reduces valve overlap and thereby-improves volumetric efficiency at low engine operating speeds. Further, the relatively early closing of the intake valves associated with such low lift short duration cams further improves volumetric efficiency at low engine operating speeds.
Therefore, what is needed in the art is an apparatus and method that enables selectively activating an engine valve according to, and selectively switching between, a high lift long duration cam and a low lift short duration cam.
Furthermore, what is needed in the art is an apparatus and method that provides a predetermined degree of valve activation for part-load engine operating conditions, and a maximum degree of valve activation for full-load engine operating conditions.
Moreover, what is needed in the art is an apparatus that enables switching between a high lift long duration cam and a low lift short duration cam, and which uses relatively few component parts and occupies approximately the substantially same space as a conventional roller finger follower.
The present invention provides a method and apparatus for two-step cam profile mode switching.
The invention comprises, in one form thereof, a two-step roller finger follower including an elongate body having a first-side member and a second side member. A first end and a second end interconnect the first and second side members. The first and second side member define first and second pin orifices, respectively. A center roller is disposed between the first and second side members. The center roller defines a shaft orifice therethrough. A shaft extends through the shaft orifice. A first shaft end is disposed proximate the first side member, and the second shaft end is disposed proximate the second side member. The second shaft end defines a shaft bore therein. The first shaft end defines a pin chamber therein. The shaft bore is substantially concentric with and intersects the pin chamber. A locking pin assembly is disposed partially within the shaft bore, the pin chamber and at least one of the pin orifices. The locking pin assembly has a first position wherein the shaft is decoupled from the body and a second position wherein the shaft is coupled to the body, and is switchable between the first and second positions.
An advantage of the present invention is that the two-step roller finger follower for cam profile mode switching enables a high liftlong duration and a low lift/short duration activation of an associated valve while occupying substantially the same amount/volume of space as is occupied by a conventional roller finger follower.
Another advantage of the present invention is that very few component parts are added relative to a conventional roller finger follower.
A further advantage of the present invention is that the two-step roller finger follower for cam profile mode switching improves engine idle quality and driveability during part-load engine operating conditions by enabling a low lift/short duration activation of an associated valve, and improves volumetric efficiency and power at high engine operating speeds.
A still further advantage of the present invention is that the two-step roller finger follower for cam profile mode switching employs roller bearings for reduced friction and increased fuel economy for both high-lift and low-lift motion.